The invention relates to an electron tube comprising a semiconductor cathode which is arranged on a support and which serves to emit electrons.
The electron tube can be used as a display tube or a camera tube but may alternatively be embodied so as to be suitable for electron lithographic applications or electron microscopy.
The electron tube can be used as a display tube or a camera tube but may alternatively be embodied so as to be suitable for electron lithographic applications or electron microscopy.
An electron tube of the type mentioned above is disclosed in U.S. Pat. No. 5,444,328. In a so-called semiconductor or xe2x80x9ccold cathodexe2x80x9d, a pn-junction is operated in the reverse direction in such a manner that avalanche multiplication of charge carriers takes place. As a result, electrodes may acquire sufficient energy to exceed the work function voltage. The liberation of electrons is facilitated by the presence of a work function voltage-reducing material.
Cesium is a work function voltage-reducing material and oxygen is a work function voltage-increasing material. Also, Cs exhibits more desorption from an O-contaminated surface. Consequently, a clean cathode surface is important. SiO is etched away from the Si cathode surface by means of HF before depositing Cs. However, in the course of the manufacture of the electron tube, said tube must be evacuated.
During said evacuation of the tube, a temperature increase is necessary in order to be able to rapidly remove the adsorbed gas to a sufficient degree from the tube walls. Many gases evacuated in said process may again lead to oxidation of the silicon cathode surface at this increased temperature (H2O, CO2, . . . ).
The invention is based on the realization that if this is not taken into account or counteracted, the emission of the cathode during operation of the tube will be lower than expected.
Consequently, it is an object of the invention to provide an electron tube comprising a semiconductor cathode which is embodied in such a manner that undesirable oxidation of the exposed (Si) cathode surface during heating of the tube (as during evacuation) is reduced.
This object is achieved by an electron tube of the type described in the opening paragraph, whereby a source is arranged in the vicinity of the cathode, preferably so as to face the free (Si) surface of the cathode, which source is capable of evolving a reducing agent at an increased temperature. A reducing agent is to be taken to mean herein a gas molecule which is capable of passivating the silicon surface at an increased temperature (as in the case of evacuation), or even of removing an oxide compound formed at the silicon surface. This process is comparable to the process step carried out in the manufacture of the cathode, in which a mixture of HF water vapor and nitrogen gas is blown from the exterior into the tube and diffused over the cathode surface, thus causing the Si surface to be passivated by hydrogen and fluorine atoms. These atoms occupy the free bonding positions of an Si atom at the surface and thereby preclude oxidation by, for example, oxygen or water vapor. It will be obvious that during evacuation, such a passivating process (using a gas flow, so-called HF gas jets) is not possible. For this reason, the invention provides a source which is capable of evolving a reducing agent at increased temperatures. The temperature during evacuation ranges in general between 20 and 400xc2x0 C., and in particular between 20 and 340xc2x0 C.
Preferably, the reducing agent comprises fluorine or a fluorine compound.
A material capable of evolving fluorine or fluorine compounds (for example HF) at an increased temperature is, for example, macor(trademark). Another material which can suitably be used is borosilicate glass or another glass capable of evolving fluorine at an increased temperature.
In accordance with an alternative embodiment, the source may be a matrix comprising a reducing agent, which agent can be readily evolved in a decelerated manner. As a result, molecules are liberated during the entire evacuation process. Said matrix may be, for example, a potassium bromide pellet. To produce this pellet, potassium bromide mixed with the reducing agent is compressed into a pellet.
The agent, whether or not comprised in a support, may alternatively be screen printed in a cell which is arranged near the cathode.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.